Recently various researches have been conducted on the growth of nitride semiconductor on a substrate made of a different material such as sapphire, spinel or silicon carbide which has a lattice constant different from that of the nitride semiconductor.
For example, a method of growing epitaxial lateral overgrowth GaN (ELOG) is described in JPN. J. Appl. Phys., vol.37 (1998), pp. L309-L312, wherein nitride semiconductor having lower density of dislocations is obtained by forming a protective film of SiO2 or other material partially on a nitride semiconductor which has been grown on the C plane of sapphire, and growing nitride semiconductor thereon under a reduced pressure of 100 Torr.
In the ELOG growing process, nitride semiconductor having reduced dislocation defects can be formed on the protective film by intentionally growing the nitride semiconductor laterally on the protective film. When the nitride semiconductor grows, dislocation occurs and grows only in a window portion of the protective film.
However, in case the protective film of SiO2 or the like has wide stripe width, lateral growth of the nitride semiconductor on the protective film does not fully proceed eventually resulting in abnormal growth.
In addition, in case the nitride semiconductor is grown laterally by vapor phase deposition process, while two nitride semiconductor films which grow laterally from the nitride semiconductor exposed on both sides of the protective film meet and join with each other at the center of the protective film, dislocations concentrate locally at the joint. This is partly due to the fact that the front surface of the nitride semiconductor is tilted while growing laterally on the protective film of SiO2 or the like. In case a device layer is formed by epitaxial growth on a nitride semiconductor substrate such as the above, microscopic pits are likely to be generated in the joint where the dislocations are concentrated. The pits are generated by the dissociation of nitrogen in the process of heating the substrate for the purpose of growing the device layer. The pits grow larger as the epitaxial growth is continued.
As a result, even when a single continuous nitride semiconductor substrate is formed by growing nitride semiconductor layer laterally on a protective film by the vapor phase deposition process, it cannot be handled in the same way as an ordinary single crystal substrate. Since the active layer of a semiconductor laser should keep clear of the vicinity of the joint, it is difficult to secure a region large enough for forming the device. Moreover, since surface of the single nitride semiconductor substrate appears to be uniform, it has been difficult to recognize the joint by viewing the top surface of the substrate and to carry out the formation of device pattern accurately.
Furthermore, in case a single continuous nitride semiconductor substrate is formed by growing nitride semiconductor laterally by using a protective film on sapphire or the like, such a structure is likely to warp. Because sapphire, the protective film and the nitride semiconductor layer, which are stacked one on another, have different coefficients of thermal expansion.
The different-material substrate may also be removed from the nitride semiconductor substrate in the last stage. The substrate of different material maybe removed by polishing or irradiating the interface between the substrate and the nitride semiconductor with excimer laser thereby breaking the chemical bond in the interface. However, it has not been easy to remove a different-material substrate such as sapphire as it takes a long time to remove by polishing or by means of excimer laser.
An object of the present invention is to provide a new structure of nitride semiconductor substrate manufactured by lateral crystal growth with a protective film, which is capable of suppressing an adverse effect caused on the device by joining the nitride semiconductor layers on the protective film. Another object of the present invention is to prevent the nitride semiconductor substrate from warping. Still another object of the present invention is to facilitate removing a substrate made of a different material from the nitride semiconductor substrate.